[clang] Skip test added in #154418 to work when the default is C++20. (PR #154463)

[Yanzuo Liu via cfe-commits]

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zwuis wrote:

We can add `-std=c++17` and change `-verify` to `-verify=expected,cxx17` so that we can use `// cxx17-warning {{...}}` instead of adding `#if` block for some test cases. See <https://clang.llvm.org/docs/InternalsManual.html#testing>.

https://github.com/llvm/llvm-project/pull/154463